228 



BOTANY 



PART I 



that the insectivorous habit is related not only to the supply of nitrogen, 

 but to that of all the nutrient salts, 



A great variety of contrivances for the capture of insects are made nse of by 

 carnivorous plants. The leaves oi Drosera are covered with stalk-like outgrowths 

 ("tentacles"), the glandular extremities of which discharge a viscid acid secretion 

 (Fig. 198 ; of. pp. 118, 119). A small insect, or even a larger fly or moth, which 

 comes in contact with any of the tentacles, is caught in the sticky secretion, and in 

 its ineffectual struggle to free itself it only comes in contact with other glands and 

 is even more securely held. Excited by the contact stimulus, all the other tentacles 

 curve over and close upon the captured insect, while the leaf-blade itself becomes 

 concave and surrounds the small prisoner more closely. The secretion is then dis- 

 charged more abundantly, and now contains an acid and a peptonising ferment 

 (p. 237). The imprisoned insect, becoming thus completely covered with the 



Fir:. 198. — Leaves of Drosera ro<u)idi/o^ia. Tliat on the left with its partly incurved tentacles is 

 viewed from above, that on the right with expanded tentacles from the side. (After Darwin, 

 enlarged.) 



secretion, perishes. It is then slowly digested, and, together with the secretion 

 itself, is absorbed by the cells of the leaf. 



In Pinguicula it is the leaf-margins which fold over any small insects that may 

 be held by the minute ejiidermal glands. In species of Utricularia (Fig. 47), 

 whicli grow frequently in stagnant water, small green bladders (metamorphosed 

 leaf-segments) are found on the dissected leaves. In each bladder there is a small 

 quadrangular opening closed by an elastic valve, which only opens inwards. Small 

 crustaceans can readily pass througli this opening, guided to it by special out- 

 growths ; but their egress is prevented by the trap-like action of the valve, so that 

 in one bladder as many as ten or twelve crustaceans will often be found imprisoned 

 at the same time. The absorjition of the disorganised animal remains seems to be 

 performed by forked hairs which spring from tlie walls of the bladder. 



More remarkable still, and even better adapted for its purpose, is the mechanism 

 e-xhibited by some exotic insectivorous ))lants. In the case of Vcnus's fly-trap 

 {Dionaea), growing in the peat-bogs of North Carolina, the capture of insects is 



